The purpose of this study is to describe the "essentiality" of polyunsaturated acids in terms of their effects upon metabolic capabilities of tissue subcellular particles. The activities of chain- elongation enzymes, fatty acid dehydrogenases, acyl transferases and fatty acid oxidases will be studies upon isomeric and homologous dienoic acids to determine the required substrate structural specificities. The activities of these enzyme systems from subcellular particles from deficient and specific fatty acid-fed rats will be measured to determine how the content of polyunsaturated acids in the microsomal structures affect metabolic capacities for lipid metabolism. Attempts will be made to fractionate and concentrate enzyme activities related to fatty acid metabolism.